Proteins studied by optical probe

A new technique to measure the compressibility of proteins accurately has been developed.

Despite their well-defined structures, proteins are fundamentally disordered. They often have strongly broadened spectra because of this disorder, and the otherwise straightforward technique of optical spectroscopy can be insensitive when applied to proteins.

Josef Friedrich and colleagues from the Technical University of Munich, Germany, have long been trying to overcome this problem. His group has now succeeded. The team used the aromatic amino acid tyrosine as an optical probe molecule in a hole burning spectroscopic study on the protein insulin.

The advantage of this technique is that the amino acid residues can be directly accessed; they are intrinsic parts of the protein under study. Previously, extrinsic labelling was necessary. Labelling can not only modify the structure and dynamics of the protein under study, it often gives information that mostly relates to the solvent and only to a lesser degree to the protein.

Friedrich expects the new approach to be widely applicable for biophysical investigations of proteins. 'The technique could be applied to a large variety of proteins since many of them contain aromatic amino acids. And since most proteins contain several aromatic amino acids (monomeric insulin for instance contains four tyrosines), it is also possible to probe local properties at several positions in the protein,' he said.

The new technique makes it possible to measure spatial variations of the protein compressibility. Being able to measure the compressibilities accurately should stimulate new work on the fundamental properties of proteins, as Friedrich explained: 'The compressibility determines the volume fluctuations and the flexibility. In addition, it determines the stability of the protein against pressure. The pressure-induced unfolding processes of proteins have attracted great interest, yet they are still far from being understood.' Philip Earis